1. Field of the Invention
This invention generally relates to small-sized step motors for use in cameras or the like, and more particularly, to a step motor suitable for use in driving a shutter, lens, barrier, and the like of a camera.
2. Description of the Related Art
In recent years, cameras have become electronics devices and shutters thereof are driven by the step motors. In addition, cameras are promoted to be small-sized and light-weighted, leading to a demand for small-sized step motors with high accuracy. For example, Japanese Examined Patent Publication No. 2-2382 discloses a step motor that simultaneously excites two coils provided on a stating element (stator) and rotates a rotating element (rotor).
FIG. 7 is a plan view schematically showing a step motor 100 disclosed in Japanese Examined Patent Publication No. 2-2382. A stator 103 having a shape of isosceles trapezoid is provided in an outer periphery of a rotor 101. The stator 103 includes three magnetic poles 104, 105, and 106, and also includes a first coil 108 on the left and a second coil 109 on the right. The rotor 101 composed of a permanent magnet is rotated by controlling the direction of the current applied to the coils 108 and 109 to change the direction of the magnetic field. In addition, the third magnetic pole 106 is magnetically excited by left and right coils, and in particular, the third magnetic pole 106 includes a protrusion portion 107 that comes close to the rotor 101. The protrusion portion 107 is provided so that a North magnetic pole (or South magnetic pole) of the rotor 101 is positioned to the protrusion portion 107 when no magnetic field is set up.
It is preferable, however, that the step motor be designed to start the rotation from a halting state with less power consumption. However, the rotor 101 of the above-mentioned step motor 100 has two magnetic poles, and it is configured that South magnetic pole or North magnetic pole is positioned to the protrusion portion 107 in a halting state. Therefore, the above-described step motor 100 has a drawback that a large amount of energy is consumed at the time of starting the rotation.